The primary document interface for the MICROSOFT EXCEL (hereinafter “EXCEL”) application is a workbook, which includes a collection of worksheets and chart-sheets (“sheets”), as shown in FIGS. 1A-B. EXCEL provides a system of tabs 101 and navigation buttons 102 that allow users to navigate a workbook. Tabs are used to select and activate sheets. They are arranged horizontally in the “tab gutter” 103 at the bottom of EXCEL's window. When the number of tabs grows beyond the tab gutter's capacity, tabs may scroll out of view. Navigation buttons are used to scroll tabs horizontally, so they may be scrolled in and out of view.
EXCEL's native navigation system has several shortcomings. First, because tabs are arranged horizontally and in one dimension, tabs prevent users from organizing workbooks logically in more than one dimension. Second, because tabs may scroll out of view as the number of sheets grows, and because the number of tabs that the user may see is limited by the bounds of the tab gutter, the full structure and content of the workbook may be partially hidden. Third, selecting sheets that are scrolled out of view often requires multiple mouse clicks to find and activate hidden tabs, creating additional effort for users and degrading the user experience.